Steel for electrical applications and novel article

ABSTRACT

A DUCTILE NON-ORIENTED ELECTRICAL-SHEET STEEL CONTAINING UP TO 0.025% CARBON, 1.5 TO 2.7% SILICON 2.2 TO 5.2% ALUMINUM WITH THE BALANCE IRON AND NORMAL IMPURITIES, AND HAVING THE COMBINED SILICON AND ALUMINUM CONTENT SUFFICIENT TO SATISFY THE EQUATION:   (PERCENT SI)+0.37 (PERCENT AL)$3.7   AND AN ALUMINUM CONTENT EQUAL TO OR GREATER THAN THE SILICON CONTENT. UPON A CONTROLLED ATMOSPHERE ANNEAL, A COMPLEX OXIDE OF IRON, SILICON AND ALUMINUM IS FORED ON THE SHEET SURFACE WHICH PROVIDES A HIGH DEGREE OF ELECTRICAL RESISTANCE ACROSS THE SURFACE OF THE SHEET.

L. J- REGITZ Sept. 25, 1973 STEEL FOR ELECTRICAL APPLICATIONS AND NOVELARTICLE Filed aanqzo, 1972 COLD ROLLAB/L/TY L/M/Z'y-COMMERC/AL EQUIPMENT5 m R h/ M wm Mm E .1 V m m WW TE w Mm ME UMV MM P m ON 6 R A 0 8 .MA nm L United States Patent ()ffice 3,7fil,253 Patented Sept. 25, 19733,761,253 STEEL FOR ELECTRICAL APPLICATIONS AN NDVEL ARTICLE 1 Lester J.Regitz, Penn Township, Allegheny County, Pa., assignor to United StatesSteel Corporation Continuation-impart of application Ser. No. 882,729,Dec.

5, 1969, now Patent No. 3,657,024, which is a continuation-in-part ofabandoned application Ser. No. 591,982, Nov. 4, 1966. This applicationJan. 20, 1972, Ser. No. 219,377

Int. Cl. C22c 37/10, 39/02 US. Cl. 75--124 3 Claims ABSTRACT OF THEDISCLOSURE A ductile non-oriented electrical-sheet steel containing upto 0.025% carbon, 1.5 to 2.7% silicon, 2.2 to 5.2% aluminum with thebalance iron and normal impurities, and having the combined silicon andaluminum content sufficient to satisfy the equation:

(Percent Si) +0.37(Percent A1)g3.7

and an aluminum content equal to or greater than the silicon content.Upon a controlled atmosphere anneal, a complex oxide of iron, siliconand aluminum is formed on the sheet surface which provides a high degreeof electrical resistance across the surface of the sheet.

This application is a continuation-in-part of application Ser. No.882,729, filed Dec. 5, 1969, Pat. No. 3,657,- 024, which was acontinuation-in-part of application Ser. No. 591,982, filed Nov. 4,1966, now abandoned.

Steel for electrical sheet applications is ferromagnetic. Ferromagneticmaterials may be divided into soft and hard, or permanent magnet,ferromagnetic categories. The soft ferromagnetic compositions are widelyused as core materials since they are capable of being magnetized tohigh magnetic field densities in electrical devices such as motors,generators, power transformers, saturable core reactors, relays andsimilar equipment, with low energy losses incurred upon magnetization.Generally, these core materials are employed in the form of punched orsheared polycrystalline sheet laminations which are stacked to form thedesired geometric shape or volume of the core.

The cores may be formed from punched, sheared or slit strips ofpolycrystalline sheet-soft ferromagnetic material that is Wound intoconcentric layers or stacked and bent to assume the desired geometricshape.

At the present time, a widely used material for electrical applicationsis a steel containing up to about 5% silicon, the balance beingsubstantially all iron plus the usual incidental steelmaking impurities.Alloys containing up to about 8% aluminum, balance iron, or up to 5%molybdenum, balance iron, have also been used for electrical-sheet steelapplications. The alloying elements are thought to be beneficial becausethey increase the electrical resistivity of the material, therebyreducing eddy currents.

In electrical-sheet steel, the crystalline structure is body centeredcubic at operating temperatures. Such materials may be easily magnetizedin a crystallographic direction parallel to a cube edge or a unit cellof the body centered cubic lattice.

For some applications, it is desirable to employ nonoriented steels,i.e., steels in which the grains have no significant alignment orpreferred orientation of their respective cube edges. Polycrystallinesheet steel satisfying this condition is characterized by having equalmagnetic induction or flux density for a given magnetizing force in anydirection in the polycrystalline sheet material, and therefore, an equalpermeability in any direction. Unfortunately, these steels which containhigh alloy content to insure low core losses are very difficult tofabricate into desirable polycrystalline sheet forms due to inability tocold roll these relatively brittle materials satisfactorily. As aresult, steels containing about 3.5 to 5% silicon are generallymanufactured into electrical-sheet material by a relatively expensiveseries of operations involving successive hot rolling and long boxannealing operations. The relatively rough surface resulting from suchhot rolling causes a decreased and inferior magnetic permeability.Moreover, many of these materials must be sheared since their excessiveroom temperature brittleness precludes economical punching for use inlaminations. In addition, the brittleness of these materials limitstheir production to relatively short sheets that can be convenientlyheated for successive hot rolling operations, or which can bemanufactured into coils by joining the relatively short sheet sectionsend to end into coil form. The difiiculties due to brittleness can beovercome by reducing the alloy content. However, while this results in amaterial that can be cold reduced and punched, such steels have a lowerquality (higher core losses) due to the increase in eddy current lossesaccompanying the decrease in alloy content.

Iron-aluminum alloys also exhibit brittleness which is believed to bedue to the presence of excessive, even when low, amounts of carbon. Itis very difficult to remove carbon in the solid state during processingof these alloys after solidification without incurring severe anddetrimental oxidation of the alloy.

It is an object of the present invention to provide a high-quality,non-oriented, polycrystalline sheet steel of soft ferromagnetic materialwhich is sufiiciently ductile to permit manufacture into coil form yetpossesses the core loss and permeability properties typical of a brittlehigh alloy electrical-sheet steel. Such steel, in accordance with theinvention, has a nonoriented body centered cubic polycrystallinestructure, a preferment of orientation not greater than that availablein non-oriented polycrystalline soft ferromagnetic materials andmagnetic properties at least as good as such available steels, and hassufficient ductility to permit cold rolling and punching.

Another advantage of my steel composition is that the magneticproperties do not deteriorate with increased sheet thickness as much asthey do with other electricalsheet steels. Thus, the core lossproperties of this steel are substantially better than conventionalelectrical grade steels at greater thickness as well.

An additional benefit of the steel composition provided in accordancewith the invention is that it lends itself to the formation on itssurface of an electrically non-conducting coating when exposed tocontrolled atmospheres during annealing operations. The coating soproduced has a superior resistance and is desirable to further reducecore loss due to eddy currents.

In accordance with the invention, there is provided a novel steelcomposition suitable for ductile, non-oriented electrical-sheet steelapplications which consists essentially of up to about 0.025% carbon,1.5 to 2.7% silicon, and 2.2 to 5.2% aluminum, the balance iron andnormal steelmaking impurities, with the combined silicon and aluminumcontent satisfying the equation:

(Percent Si) +0.37(Percent Al)53.7

and the aluminum content being equal to or greater than the siliconcontent. Another embodiment of the invention comprises a sheet of theaforementioned steel composition containing a surface coating of acomplex iron-aluminum-silicon oxide, preferably less than 1 mil thick.Uninterrupted coatings in accordance with the invention of less than0.0005-inch improve the surface resistance substantially.

the total alloy content, silicon plus aluminum, does not exceed therelationship:

(Percent Si) +0.37(Percent Al) 3.7

In summary, therefore, most of the above composition limits are verycritical if the sheet is to have sufficient ductiltiy to permit coldrollability on commercial equipment, and yet retain commerciallyacceptable magnetic properties. Specifically, in order to obtain theessential degree of ductility, it is critical that (1) the total alloycontent, silicon and aluminum, must satisfy the above noted equation;(2) the silicon content must not exceed 2.7%; and (3) the aluminumcontent must not be less than the silicon content. Moreover, in order toobtain magnetic properties equal to or superior to prior art commercialsteels, the silicon content must be at least 1.5%. Since the aluminumcontent cannot be less than the silicon content, it is thereforeessential that the aluminum content must also be at least 1.5% formaterial to be cold rolled on laboratory mills, and at least 2.2% toyield sheet material with acceptable magnetic properties after coldrolling on production facilities. And, as the prior art has shown, thecarbon content must be minimized in order to optimize magneticproperties. The attached figure is a Fe-Si-Al ternary phase diagramillustrating the above compoition limits as shown in the shaded area.

The resulting ductility effected by this invention is greater than theductility of iron-silicon alloys of comparable alloy content. Theincrease in ductility is accomplished without impairing resistivity andsaturation magnetization, and at the same time initial and maximumpermeabilities increase. It is apparent that greater ductility is animportant consideration in cold Working these steels and the workingsignificantly affects the final magnetic properties, particularly thecore loss and permeability at given induction. Moreover, cold working ismore economical than hot working here. A preferred composition inaccordance with the invention contains up to about 0.015% carbon, 2 to2.7% silicon, 2.2 to 4% aluminum and a combined silicon and aluminumcontent of between 4.2 and 6.7%, the balance substantially iron andnormal impurities.

In the preferred method of fabricating steel of the abovementionedcompoistion into sheets and coils suitable for the manufacture ofelectrical equipment components, clean ingots of the composition arefirst brought to temperature of between about 2100 and 2500 F., moredesirably between 2200 and 2350 F., by soaking and rolled to slabs,usually less than 8-inches, i.e. between 5- and 7-inches thick, at afinishing temperature between 1600 and 2000 F., preferably 1700 and 1800F. The slabs may be slow cooled if desired or if necessary due to theslab thickness. The slabs are reheated to 2100 to 2500 F., preferably2200 to 2350 F., and hot rolled to sheet having a thickness between 0.06and 0.12 inch at a finishing temperature of 1600 to 1800 F. Thehot-rolled sheet is water cooled by spraying to a temperature belowabout 1700 F., and may be cooled to room temperature.

Hot-rolled sheets so produced may then be pickled to remove scale andcold rolled to final thickness directly and without intermediateannealing, if desired. Conventional processing may also include anannealing after pickling, prior to cold reduction. The cold reducedsheet may then be box annealed or continuously annealed in appropriatefurnaces at between 1400 and 2100 F., a preferred temperature between1600 and 2000 F. is desirable, depending upon the mode of annnealing.The annealing time at temperature may vary although generally annnealingis performed for a time sufficient to promote recrystallization, somegrain growth when desired and some purifiction.

An improvement in the aforementioned manufacturing process to produce aproduct in accordance with the invention involves performing theaforementioned final annealing in a hydrogen or nitrogen atmosphere, ormixtures thereof, or any inert gas atmosphere with a dew point between+30 and 26 F., and preferably less than 8 F. The dew point may beadjusted, however, to allow some purification, particularly of carbon,while not allowing appreciable oxidation of the alloy, particularlyoxidation of the aluminum contained in the alloy. By using atmospherecontaining a limited quantity of water vapor as measured by the dewpointof the gas, a continuously coherent and adherent coating composed ofalumina, silica and iron oxide is produced on the surface of the sheetwhich provides a superior level of resistance to the fiow of electricalcurrent through the surface of the sheet.

The following examples will aid in understanding the invention:

Steel containing 1.5 to 5.2% aluminum, 1.5 to 3.5% silicon and thebalance substantially iron and the normal steel-making impurities of thecompositions shown in Table I were vacuum melted by processes whichsimulated melting in open hearth or basic oxygen steelmaking facilitiesfollowed by vacuum carbon deoxidizing and then cast into 3 x 8 x 14-inchingots. After reheating to a temperature between 2200 and 2350 F., theingots were hot rolled to l-inch slabs at a finishing temperature ofbetween 1700 and 1750 F. and slowly cooled in insulating vermiculite.After conditioning and reheating to 2250 F., the slabs were hot rolledto a thickness of 0.080-inch at a finishing temperature between 1600 and1700 F. Following hot rolling, the sheet was cooled either by quenchingto room temperature in a water spray or by quenching to a temperaturebetween 1250 and 1300 F. accomplished by holding in a furnace at 1200 F.for 1 hour. The latter process is preferred to simulate commerciallyemployed coiling practice. The samples were then pickled, cold rolled to0.014-inch, sheared and annealed at the various times and temperaturesshown in Table II.

Sample numbers 1 through 12 were laboratory samples processed asdescribed above. Samples 13 through 16 were of commercially producedAISI grades of electrical sheet steel, and are included for comparison.Samples 17 and 18 were samples taken from the product of a 30-ton heatmade on commercial production facilities.

TABLE I Sample numbers r AISI M14 grade.

2 AISI M15 grade.

S Si Cu AISI M17 grade.

3 Maximum. AISI M19 grade.

TABLE II.SAMPLE DESIGNATION AND PROCESSING TREATMENTS EMPLOYED IN THEIRPRODUCTION Nominal composition, Annealing treatment percent Tempera- AlSi Treatment prior to final anneal Time ture, F. Atmosphere 1. 5 3. 25Hot rolled, quenched, cold reduced 1 10 minutes 1,800 N -15% Hz 2 3 dodo 1,800 N215% H2 3 do 1, 800 N215% Hz 4 2 .....d0. do 1,800 Nr15% Hg 32 Hot rolled, coiled, cold reduced 1 do 1,800 Ive-15% H2 4 2 Same(including footnotes) 1, 800 N215% H;

1.5 .3. 25 do 1,800 Hz 3 2 1,800 H2 1. 5 1,600 H215% Na 2 1,600 N215% Hz3 2 1,600 N215% H2 4 2 ..do. --do 1,600 N-r15% H2 4. Hot rolled,commercial M14 non-oriented Hot rolled 0 4. 25 Hot rolled, commercialM15 non-or ented do 0 4 Hot rolled, commercial M17 non-oriented..-

2. 5 2. 5 Hot rolled, slow cooled, cold reduced 4 minutes 1, 830 N;

2.5 2.5 do. 3minutes 1,900 N;

1 Single cold reduction.

2 Double cold reduction with intermediate anneal. 3 Simulated by slowcooling from 1,200 F to room temperature.

Annealing was accomplished in either nitrogen, hydrogen or a mixednitrogen atmosphere containing 3 to 15% hydrogen and having a dewpointbetween about +6 and --26 F. as shown in Table III.

tion Test. The coating was obtained by annealing under the conditionsshown in the table for each sample. However, the surface coating couldbe developed through anneals at lower temperatures and shorter timeswith appropriate increases in the dewpoint of the atmosphere.

TABLE IV MAGNETIC PROPERTIES OF SAMPLES Maximum 10 kg. kg. torque,Franklin thousands insulation Core loss, Core loss, of dyne- Ampere atw./lb./60 Perm. w./lb,/60 Penn c1n./cm. 250 psi 0.620 7, 750 1.39 1,20016 e 0.05 0.645 7, 500 1.47 1, 145 20 0.016 0.580 8, 300 1.80 1, 220 13l 0.09 0.640 6,000 1.49 450 22 0. 02 0. 640 7, 500 1. 44 918 ND N D 0.647 2, 875 1.60 215 ND ND 0. 643 6,150 1. 46 892 ND ND 0.625 5,850 1.401,015 ND ND 0.51 8,100 1.10 1,064 25 ND 0. 57 7, 576 1. 26 938 23 ND 0.59 5, 100 1. 32 857 19 ND 0. 62 5, 025 1. 31 714 20 ND 0. 532 3, 409 l.22 378 37 b 0. 56 0.549 4, 379 1. 29 377 49 b 0. 5O 0. 602 4, 456 1. 34412 57 b 0.47 0. 519 6, 897 1. 25 600 44 0. 56 0.470 7, 937 1. 1, 230 34d 0.25

a After annealing at 2,000 F. in Nz-3% Eli-8 F. dew point, for 8 hours.

b Separate core plating and annealing operations.

v After annealing at 1,830 F. in nitrogen with a 26 F. dew point.

d After annealing at 1,900 F. in nitrogen with 2. +20 F. dew point.

No'rE.ND=Not determined.

TABLE IIL-DEWPOINT RANGES IN EXPERIMENTAL ANNEALS Dew point Temperature,F. Length of time range Magnetic properties are not alwayswell-developed in anneals conducted at a dewpoint of +6 F., but thesurface coating was satisfactory. Dewpoints of less than about -8 F.consistently produce superior coatings. The sheets thus produced weretested for core loss and permeability at 10 and 15 kg. and at 60 cyclesand the resulting values corrected for the specific gravity of thesamples being treated. The corrected data for successful combinations ofcomposition and processing are shown in Table IV along with the resultsof typical torque magnetometer data and data reflecting the resistanceof the surface coating as measured by the well-known Franklin Insula-Annealing can be accomplished at any temperature between 1400 and 2200F. for a period of 3 minutes to 8 hours as necessary to develop suitableelectrical properties. The preferred temperature is 1600 to 2000 F. withthe optimum conditions being between 1800 and 1900 F. for 3 to 6 minutesfor continuous anneal or for 8 hours for a box annealing. Alloysdeveloped effective coatings at all annealing times and temperaturesused within the dewpoint range of +3 to -26 F.

As mentioned above, steels in accordance with the invention exhibitsubstantial superior core loss properties in thicker sheet sizes thanconventional electrical grade steels. The data in Table V, for example,shows how the core loss properties decrease (i.e. increase in value)with sheet thickness. Even at thicknesses of 18 and 25 mils the coreloss of steel in accordane with the invention is almost 50% less thanthe other steels. Sample 2A corresponds in composition to sample 2 ofTable I. However, it was treated by double cold reduction withintermediate and final anneals in an atmosphere of 15 H in N for 10minutes at 1800 F.

TABLE v.-10 KG. CORE LOSS VARIATION WITH 1 Not available.

All samples containing from 1.5 to 3.5% silicon and 1.5 to 6% aluminumwhere the total content was from 3 to 7.5%, yielded the desired magneticproperties and ductility when processed with laboratory equipment whereno end-tension is applied. However, when experimentation was advanced tocommercial production equipment, more limiting critical compositionlimits were shown to be necessary. Specifically, some compositionswithin the above range cannot be cold rolled on commercial equipmentwithout breaking, despite extreme precautions to prevent breakage.Evaluation of these experiences revealed that in order to insuresufficient ductility to be cold rollable on commercial productionequipment without breakage, it is necessary to confine the siliconcontent to the range 1.5 to 2.7%, and to assure that the aluminumcontent is equal to or greater than the silicon content. Therefore, eventhough the samples within the first discussed specified ranges doprovide superior magnetic properties and improved ductility, theductility is still not sufficiently improved to permit commercial coldrolling unless the silicon is below 2.7%, i.e. Within the range 1.5 to2.7% and the aluminum to silicon ratio is at least unity.

To understand the differences realized between laboratory cold rollingand commercial cold rolling, it must first be noted that additions ofsilicon and/ or aluminum to the steel causes a reduction of bothductility and malleability. Both are reduced sufficiently to limit thealloy content of cold-rollable iron-silicon-aluminum alloys. The effectof aluminum additions on such reduction of cold-rollability is not aspronounced as the effect of silicon additions, as is evidenced by lineson the attached figure, representing alloy limits of cold-rollability inthe laboratory and on production facilities, which indicate higher totalalloy contents, and hence superior magnetic properties for alloys inwhich aluminum content is high, relative to silicon content. With thisin mind, it is noted that laboratory coldrolling is performed on smallsample sheets without end tensioning, hence, malleability is the primarycontrolling factor. In commercial cold-rolling, however, the equipmentincludes end-tensioning means applied to the strip being rolled tofacilitate easier cold-reduction and to provide precise, tight and neatcoils of cold rolled steel. On commercial equipment, therefore,malleability and ductility are both factors. The effect of reduction ofductility is additive to the effect of reduction of malleability causingthe total alloy content to be reduced for cold-rollability on productionfacilities, relative to such limits for laboratory facilities. Alsoappearing in the attached figure are minimum alloy contents permittingthe superior magnetic properties displayed by alloys of this invention.

It is apparent from the above that various changes may be made withoutdeparting from the invention. Accordingly, the scope thereof should belimited only by the appended claims.

I claim:

1. A ductile electrical sheet steel having a non-orientedpolycrystalline structure consisting essentially of up to about 0.025,%carbon, 1.5 to 2.7% silicon, 2.2 to 5.2% aluminum, the combined siliconand aluminum content satisfying the equation:

(Percent Si) +0.37(Percent A1) 53.7

with the aluminum content being at least equal to the silicon content,and the balance substantially iron and normal impurities.

2. A steel according to claim 1 in which the carbon content is belowabout 0.015%.

3. A ductile electrical sheet steel having a non-orientedpolycrystalline structure consisting essentially of up to about 0.025%carbon and silicon and aluminum contents within the shaded area in theattached figure.

References Cited UNITED STATES PATENTS 842,043 1/ 1907 Hadfield -l242,193,768 3/1940 Masumoto 75--124 HYLAND BIZOT, Primary Examiner

